Because of their physical and chemical stability, nitride films are widely used in the process of fabricating semiconductor integrated circuit devices. Nitride films typically have better mechanical strength, vapor barrier characteristics and sodium barrier characteristics than, for example, oxide films. However, it is often difficult to reliably etch nitride films.
In many cases, nitride films must be selectively removed with respect to oxide films. In a conventional etching process, a nitride film is selectively etched with respect to an oxide film using phosphoric acid (H3PO4). Specifically, a semiconductor substrate on which a nitride film and an oxide film are formed may be placed in a phosphoric acid bath, and the bath may be heated at a temperature of approximately 160 to 170° C. In this case, an etch rate of the nitride film may be approximately 40 to 45 Å/min, and that of the oxide film may be approximately 1.2 to 2.0 Å/min. That is, selectivity for nitride to oxide at a temperature of approximately 160 to 170° C. is approximately 26 to 27.
However, due to its high viscosity, phosphoric acid may have to be preheated for a lengthy duration to reliably reach the temperature of approximately 160 to 170° C. In addition, relative poor stability of phosphoric acid may require at least two dummy etching cycles before the etching process using phosphoric acid is actually performed. Etching processes using phosphoric acid may also be relatively expensive.